Here is a 4′ tall book shelf used as a room divider. This was made out of MDF. I built a half wall and sheet rocked it. The sheet rock acted as the back of the book shelf.
A 1″x4″ pine was used for the bottom face. Regular 3 1/4″ trim was used for the base. The vertical front faces were made out of pine 1″x2″ ‘s. The adjustable shelves holes were drilled every 1 1/2″. The shelves are MDF with rounded over edges. The top horizontal face is a 1″x6″ pine, with crown molding wrapping around it. The top was cut to hang past the crown mold a 1/2″. MDF must be painted with an oil base Kilz for the primer. Then paint over it with an acrylic/latex paint.
This is the back side of the book shelf. The top was probably 24″ wide at least.